In ocean tankers carrying cargoes of liquid natural gas (LNG), as well as land based storage tanks, a portion of the liquid, normally amounting to approximately 0.1 to 0.25% per day in the case of LNG, is lost through evaporation as a result of heat leak through the insulation surrounding the LNG storage receptacle. Moreover, heat leakage into LNG storage containers on both land and sea causes some of the liquid phase to vaporize thereby increasing the container pressure.
Shipboard LNG storage tank boil-off has typically been used as an auxiliary fuel source to power the ship's boilers and generators. However, recent LNG tanker designs have incorporated the use of diesel engines rather than steam driven engines thereby eliminating the need for supplemental energy supplied by LNG boil-off.
Recently enacted legislation prohibiting tanker disposal of hydrocarbon-containing streams by venting or flaring within the vicinity of metropolitan areas coupled with an increased desire to conserve energy costs have led to incorporation of reliquefiers into the design of new tankers for recovering LNG boil-off.
Attempts have been made to recover nitrogen-containing natural gas boil-off vaporized from a storage tank. Typically, these systems employ a closed-loop refrigeration system wherein cycle gas is compressed, cooled and expanded to produce refrigeration prior to return to the compressor. The following patent is representative:
U.S. Pat. No. 3,874,185 discloses a reliquefaction process utilizing a closed-loop nitrogen refrigeration cycle wherein the lowest level or coldest level or refrigeration for condensation of LNG is provided by an isentropically expanded stream while the remaining refrigeration is provided by isenthalpic expansion of the residual second fraction of refrigerant. In one embodiment, the residual fraction of the isenthalpically expanded stream is subjected to a phase separation wherein liquid and vapor fractions are separated. During periods of low refrigeration requirements a portion of the liquid fraction is stored, and, during periods of higher refrigeration requirements, a portion of the stored liquid fraction is recycled into the refrigeration system.